Development and Validation of a New Eddy Dissipation Concept (EDC) Model for MILD Combustion

Abstract

Moderate and Intense Low-oxygen Dilution (MILD) combustion is established to achieve high thermal efficiency and low pollutant emissions, including both NOx and soot. The Eddy Dissipation Concept (EDC) is the most widely used combustion model in the existing numerical studies of turbulent MILD combustion. It is easy to implement and has acceptable computational cost. EDC is a turbulence-chemistry coupling model which assumes that the reaction zone can be modelled as a well-stirred chemical reactor, exchanging mass with the environment at a rate determined by turbulence. However, the low Damköhler number found within the homogeneous reaction zone brings a challenge. The standard EDC model is found to predict too early ignition. A simple solution is to modify the model constant parameters Cτ and Cγ to a different value using the experimental data, which is not effective since the method is case dependent. Recently, Parente et al. proposed an extension of EDC model which qualitatively calculates the model constants locally, depending on the local turbulent Reynolds number and Damköhler number. The new extended EDC model used in this work is a further development of Parente et al.'s model. We improved the model by assuming the chemical time scale is the time needed to traverse the fine structures (T = L* /SL) for both Cτ and Cγ, In this way, the model is able to quantitatively define Cτ and Cγ locally without any tuning. Besides, Cγ is found to be proportional to Da*3/4 rather than Da*1/2 in Parente's model. The new model is validated through the Delft-jet-in-hot-coflow (DJHC) burner database and further applied in a laboratory-scale MILD furnace in order to give theoretical insight. For the DJHC burner case, the RSM turbulence model is proved to give better agreement compared to the widely used modified standard k-ε model. The new extended BOC model is validated in terms of temperature, flow, and OH-concentration-based liftoff height. The temperature peak is captured better. A study of influencing factors, including the jet velocity, fuel temperature, and temperature and oxygen concentration of coflow, is undertaken. The influences of these issues on flame volume, liftoff height and peak temperatures are analysed. For the furnace case, the prediction of the new extended EDC model is compared with the EDC model with modified constant parameters. The new model can provide a comparable prediction compared to the widely used model. The analyse of NOx shows the maximum NO concentration is lower than approximately 10 ppmv in the furnace. The thermal NOx formation process is not dominant in the furnace studied.